Such collision warning devices are also referred to as PSS systems (predictive safety systems); their purpose is to call the attention of the driver to an imminent collision danger, for example by an acoustical signal, and/or to actively trigger emergency braking or other measures to avert the collision or to mitigate the consequences of the collision.
This is done by interpreting the data from a locating sensor, for example an angular resolution radar sensor, which makes it possible to measure the distances, relative velocities and azimuth angles of preceding vehicles. To assess the danger of collision, a danger parameter is calculated. This may be, for example, the so-called “time to collision” (ttc), which is calculated in advance under the assumption that the vehicles involved will maintain their present velocity or acceleration state. If the ttc drops below a certain threshold value (so that it passes the threshold in the direction of smaller values), a warning is triggered. Alternatively, the (negative) acceleration that would be necessary to avoid the collision may also be used as the danger parameter. An algebraic or logical function of the two parameters named above may also be used as the danger parameter.
PSS systems are frequently offered in combination with so-called ACC systems (adaptive cruise control), which serve to regulate the velocity of the vehicle in such a way that a vehicle traveling immediately ahead in the same lane is followed at an appropriately safe distance.
Based on the distances and azimuth angles measured by the radar sensor, the cross track distances of the located vehicles may be calculated, so that it is possible to decide whether a vehicle is in the same lane or an adjacent lane. At the same time, it is possible to determine in this way how many lanes the currently traveled road has and in which lane the host vehicle is traveling.
A collision warning system is described in German Patent Application No. DE 101 03 401 A1 in which a prevailing driver stress level is calculated from vehicle status variables, surroundings data and data about the driver's activity acquired from a sensor system, the driver stress level then providing the basis for the warning strategy. The surroundings data mentioned there also include the number of lanes of the road. However, there is no further explanation of how the number of travel lanes affects the driver stress level.
The choice of the threshold value for triggering the warning signal or warning always requires a compromise. On the one hand, the triggering threshold should be as low as possible, so that a warning is triggered as early as possible and enough time remains for the driver (or the system) to respond and avert the collision. On the other hand, a lower triggering threshold increases the frequency of false warnings, which the driver may find irritating or at least tiresome, and which, if they are too frequent, may result in the driver shutting off the system altogether.
Especially problematic in this connection are PSS systems that react not only to moving objects, for example other moving vehicles, but also to stationary objects. Since stationary objects by the roadside occur relatively frequently, and since due to uncertainties in predicting the course of the route it is not always possible to decide with certainty whether the object really constitutes a relevant obstacle or is located by the roadside or at least away from the anticipated travel path of the host vehicle, the danger of false warnings is especially great here.
For example, when the moment of warning is too early, false warnings may occur frequently if a traffic sign or the like standing by the roadside is located before the vehicle enters a curve and is erroneously interpreted as an obstacle. In practice therefore, up to now a high triggering threshold and a correspondingly late warning moment have been chosen for stationary objects. This greatly limits the usefulness of the system, however, since only a correspondingly short advance warning period then remains, even in situations of genuine danger.